Internalizing disorders (depression and anxiety) in childhood have been the focus of recent research interest, in part because of converging evidence that these childhood difficulties predict both concurrent and later behavioral and functional impairments (Hammen &Rudolph, 2003). Theoretical models and empirical work on the development and maintenance of depression in childhood have pointed to negative social experiences and the absence of positivity in social interactions as important and potentially even causal co-occurrences. Studies have explored social interactions between depressed children and their parents (especially mothers) and peers. However, surprisingly little attention has been paid to the sibling relationship. 80% of American children grow up in a household with one or more sibling (Dunn, 2000). These relationships are known to be intense and highly affectively-charged. The current study will examine early childhood sibling interactions as a predictor of later internalizing problems with a focus on examining how positive sibling interactions protect against subsequent psychiatric difficulties in a high-risk, longitudinal sample. Thus, the current study will explore an understudied relationship (sibling) in a high- risk group (low-income children), utilizing observational methodology, to advance our understanding of the developmental pathways to a significant mental health problem. This work is intended to address the NIMH strategic objective of defining and identifying risk and protective factors for the development of internalizing disorders in childhood and adolescence. This work is intended to help both clinicians and treatment researchers who are working with children and families that are struggling with depression and anxiety. By understanding mechanisms in interpersonal relationships that can both exacerbate and protect against risk for depression and anxiety we can learn how and when to intervene to promote mental health in high-risk populations. PUBLIC HEALTH RELEVANCE: While there has been much interest in the interpersonal contexts of childhood depression and anxiety ("internalizing") disorders, relatively little research has examined the role that sibling interactions may play in development of mental illness, especially internalizing disorders. The current study will examine sibling interactions in a low- income sample to see if positive sibling interactions help protect children at risk for developing depression and anxiety. This work is intended to inform prevention and intervention approaches for children at risk for internalizing disorders and could be used to help develop family-based interventions.